A heightened awareness of the environmental side effects of manufacturing has led industry to make some remarkable strides in reducing the amount of industrial waste produced as a by-product of the production of items of manufacture. Historically the manufacture of paper has been thought of as energy intensive and requiring large amounts of water. Nonetheless, the paper industry has developed techniques for recycling the water used in the production of paper to the point that the demands papermaking makes on water resources have been substantially reduced.
At the same time that remarkable strides have been made in the papermaking industry's impact on water resources, paper itself has been found to be an economically recyclable material. Already a significant portion of the fiber used in manufacture of cardboard, certain grades of linerboard and newsprint are manufactured mainly or completely of recycled paper fibers. Unfortunately, paper cannot be recycled indefinitely. Each time the paper fibers are recycled, and even when the fibers are first manufactured from wood chips, a certain percentage of the fibers are broken into such small pieces that they cannot be used to form paper. These small particles of wood fibers must be separated from the wastewater stream to allow the wastewater to be reused. The separated product is known as sludge.
Sludge is divided into primary sludge and secondary sludge. Primary sludge, which typically is seventy percent of the sludge produced, consists of the larger particles of wood fibers. These particles may be separated from the wastewater stream by allowing the wastewater to stand in a clarifier where the heavy waste particles separate by settlement. Secondary sludge is comprised of those particles which are too small to settle in any reasonable length of time from the wastewater stream.
Recycling paper increases the amount of both primary and secondary sludge produced in the papermaking process. As the paper fibers are reused the percentage which are broken into pieces too small for use in manufacturing new paper increases. Secondary sludge consists of colloidal and colloidal-sized particles. The secondary sludge may be precipitated in a clarifier by use of a flocculent to precipitate the secondary sludge particles by the process of coagulation, that is, by binding together the minute particles to form flocs which are of sufficient size to precipitate in a clarifier.
Primary sludges are readily dewatered by mechanical means such as by screw press, see for example U.S. Pat. No. 4,582,568 to lyengar, or by belt presses. Dewatered primary sludges may be dried in a rotary drum dryer, for instance, as shown in U.S. Pat. 5,207,009 to Thompson, et al. The dried primary sludge can then be used as a boiler feed to produce energy for use in the papermaking process.
Secondary sludge, however, has the consistency of gelatin and mechanical dewatering means are largely ineffective. Limited dewatering of secondary sludge has been achieved by blending the secondary sludge with the primary sludge, thus adding fibrous material which is contained in the primary sludge to the secondary sludge. The fibrous material provides limited bonding between the flocculated particles and the fibrous material which facilitates, to a limited extent, the dewatering by mechanical means of the combined sludges. Similarly, but perhaps with more success, pulped waste newsprint fibers have been mixed with phosphatic clays for dewatering them as suggested by R. F. McFarlin, et al. in Benefication of Phosphate: Theory and Practice, pp. 429-446 (SME, 1993).
Without effective dewatering of the secondary sludge, disposal of the secondary sludge presents a problem, a problem of increasing urgency as the use of recycled fiber increases. Drying the secondary sludge with a higher water content is energy-intensive and expensive. Landfilling the sludge is similarly undesirable from an expense and environmental viewpoint.
What is needed is a method of dewatering secondary sludge produced in the papermaking process.